OP, their photos conflict.
I've now looked at GSV and can see that the lower part of the actual sign says: Loading only 7am - midday. In the CEO's first photo which is reproduced in their rejection the '7am-midday' is omitted, hence my earlier comment. However, their response also includes a photo showing the sign in full. They really don't know what they're doing, but what's new?
But in any event, there isn't a single yellow line with loading blips within and either * at the rear or at the front of the parking place to give effect to the no waiting/no loading restriction. The sign is insufficient and the absence of the markings is fatal to their case IMO.
I would carry on, but this is a decision for the registered keeper. Is this you and are your DVLA details current?